Light source configured for stabilization relative to external operating conditions

ABSTRACT

A laser device includes a light source that emits a source light having a first peak wavelength. A nonlinear optical component performs a frequency conversion process that converts the source light into output light having a second peak wavelength. A stabilization component minimizes a mismatch error constituting a difference between the first peak wavelength and a wavelength for which the frequency conversion process in the nonlinear optical component has a maximum value. The stabilization component may include a housing that is thermally conductive between the light source and the nonlinear optical component to minimize a temperature difference between the light source and the nonlinear optical component. The laser device may include a focusing optical component that focuses the source light to have a convergence half angle that is larger than a convergence half angle that gives maximum output power, thereby increasing an acceptable range of the mismatch error.

TECHNICAL FIELD

This invention relates to a light source device usingfrequency-conversion of light from a laser light source for exemplaryuse to provide a deep ultraviolet light source device which has a wideoperable range with respect to changes in operating conditions.

BACKGROUND ART

There are many examples of conventional photonic devices which exploitnonlinear optical properties of materials to convert light of a firstfrequency to light of a second frequency. Common examples exploit thenonlinear optical properties to provide second harmonic generation (SHG)whereby light with a first frequency emitted by a laser source (a “pump”laser) is converted to light with a second frequency which is double thefirst frequency. This process is commonly referred to asfrequency-doubling. The frequency-doubled light is laser-like, meaningit has many features similar to the characteristics of light emitted bylasers such as narrow range of wavelengths, high beam quality and stronglinear polarisation. Photonic devices which emit frequency-doubled lightare also often referred to as lasers or frequency-doubled lasers.Frequency-doubling is often used to provide lasers with emissionwavelengths which are difficult or impossible to achieve using directlasing.

The efficiency of frequency-doubling can be sensitive to changes inconditions including the temperature of the frequency-doubling nonlinearoptical material and the wavelength of input light. This sensitivityoften necessitates use of components to actively stabilise thetemperature of one or more components in frequency-doubled lasers whichare deployed in situations where the ambient temperature or otherconditions may vary. One important category of frequency-doubled lasersare those configured to provide emission of deep ultraviolet (UV) light(that is, light with wavelength between ≈200 nm and ≈300 nm). There issubstantial demand for compact, high performance and low-cost lightsources of deep UV light, and especially for deep UV light lasers orlaser-like sources. The demand is high because deep UV light may be usedfor efficacious chemical-free disinfection of bacteria and viruses andto enable sensors for chemical or biological compounds owing tocharacteristic fluorescence, absorption or scattering of the deep UVlight. There are no laser diodes available at deep UV wavelengths.

There are examples of conventional devices for generating deep UV lightusing frequency-doubling of the light emitted by laser diodes. Nishimuraet al. [Japanese Journal of Applied Physics 42 p 5079 (2003)] describesa system to frequency-double an input beam with a wavelength of 418 nmusing a bulky and complex optical resonator structure to generate anoutput wavelength of 209 nm. Tangtrongbenchasil et al. [Japanese Journalof Applied Physics 45 p 6315 (2006)] describes a system tofrequency-double an input beam with a wavelength of 438 nm using anotherbulky, complex design with a temperature controller applied directly tothe nonlinear optical component (a β-BaB₂O₄ crystal) to hold thecomponent at a stable temperature to generate 219 nm wavelength.Tangtrongbenchasil et al. [Japanese Journal of Applied Physics 47, p2137 (2008)] describes a system to frequency-double an input beam with awavelength of 440 nm from a laser diode using another bulky, complexdesign in which the temperature of the laser diode is stabilised using athermoelectric cooler (TEC), which nonetheless yields an output offrequency-doubled light (wavelength 220 nm) with very low optical power(≈200 nW). Ruhnke et al. [Optics Letters 40, p 2127 (2015)] describes asystem to frequency-double an input beam with a wavelength of 445 nm togenerate a 222.5 nm wavelength output using another bulky, complexdesign, including an oven to assure stable temperature of the nonlinearoptical frequency-doubling component (a β-BaB₂O₄ crystal) at 50° C.Other features for frequency-doubled lasers capable of emitting deepultraviolet light using laser diode pump lasers are disclosed in U.S.Pat. No. 8,743,922B2 (Enescu et al., issued on Jun. 3, 2014) andUS20150177593A1 (Smeeton et al., published on Jun. 25, 2015).

SUMMARY OF INVENTION

This invention provides a device for generating light through frequencyconversion of source light emitted from a laser light source and whichis effective over a wide range of operating conditions without the needfor active temperature stabilisation of the device. The presentinvention enables for the first time the manufacture of laser deviceswhich are compact and low cost, at wavelengths where such devices havenot previously been realised (e.g. deep ultraviolet wavelengths). Inconventional devices it is anticipated that a device which frequencyconverts laser light, particularly if the output wavelength is in theultraviolet spectral range, will only maintain an effective power outputover a narrow range of operating conditions. This arises because of awavelength mismatch error between the wavelength of the source light andthe wavelength which the nonlinear optical component will frequencyconvert with good efficiency. Therefore, there has previously been theneed to carefully control the operating conditions. For example, thelaser light source and/or the nonlinear optical component may beactively temperature controlled. However, this adds complexity and sizeto the overall device. This invention provides a means of removing theneed for such careful control of the operating conditions while stillproviding a device which maintains an effective power output.

In an aspect of this invention the wavelength mismatch error is reducedby configuring the laser light source and the nonlinear opticalcomponent such that the wavelength of the source light emitted from thelaser light source and the wavelength which is frequency converted withgood efficiency exhibit a similar change for a change in operatingcondition.

In an aspect of this invention the laser light source is configured suchthat the wavelength of the source light emitted by the laser lightsource is stabilised against changes in external operating conditions.This reduces the wavelength mismatch error in situations where thewavelength of the source light changes significantly more than thewavelength which is efficiently frequency converted for the same changein operating condition.

In an aspect of this invention the laser light source and nonlinearoptical component are attached to a housing which is configured toprovide good thermal contact between the laser light source andnonlinear optical component. This reduces the wavelength mismatch errorwhen operating conditions are changed.

In an aspect of this invention source light which does not undergofrequency conversion is used to heat the nonlinear optical component.This may reduce the wavelength mismatch error when the electricalcurrent supplied to the laser light source and/or the operating mode ofthe laser light source are changed.

In an aspect of this invention the device is configured such that thesource light incident on the nonlinear optical component has aconvergence which provides a large acceptable wavelength mismatch errorwhile still allowing the device to be effective. In particular, thepresent inventors have found that the convergence angle which providesan advantageously large acceptable wavelength mismatch error differsfrom the convergence angle taught in the prior art for achieving thehighest efficiency frequency conversion. In situations where a device isrequired to be effective over a certain range of operating conditions,it is advantageous to move away from the convergence angle taught in theprior art.

In an aspect of this invention the laser light source is configured toemit light with a broad spectral linewidth. This increases theacceptable wavelength mismatch error and therefore provides a devicewhich is effective over a wider range of operating conditions comparedto a device which uses a laser light source which is not configured toemit laser light with a broad spectral linewidth.

An unanticipated new problem arising from the advantageously largeoperable range provided by aspects of this invention is that thedirection and/or position of the frequency converted output light maychange when the operating conditions are changed. Therefore, in afurther aspect of this invention this change in direction and/orposition is reduced by the inclusion of one or more additional opticalcomponents.

Any or all of these aspects may also be combined. The aspects of thisinvention enable a frequency converted light source which:

-   -   remains effective over a wider range of operating conditions        than a device which does not use these aspects.    -   is lower cost than a device which does not use these aspects        because of the relaxation in the control required over the        operating conditions (e.g. active temperature stabilisation is        not required, thus reducing component number and complexity).    -   can be more compact than a device which does not use these        aspects for the same reason as above (e.g. active temperature        stabilisation is not required, reducing the number of components        and therefore the size of the device).

The invention is particularly advantageous for devices which generatedeep ultraviolet light (e.g., wavelength between ≈200 nm and ≈300 nm) bysecond harmonic generation using a laser diode to generate the sourcelight. Demand for light sources which are capable of emitting deepultraviolet light is increasing and in many applications the capabilityto remain effective over a wide range of operating conditions is eitherhighly desirable or essential.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary laser device according to an aspect of thisinvention.

FIG. 2(a) and FIG. 2(b) show the relationship between

_(peak),

_(PM) and α as the temperature of the laser light source and thenonlinear optical component vary (a) according to an aspect of thisinvention and (b) not according to an aspect of this invention.

FIG. 3 shows the effect on α of configuring the laser light source suchthat the wavelength of source light is stabilised against changes inexternal operating conditions.

FIG. 4 shows a relationship between the laser light source, thenonlinear optical component and the heat sink with respect to thethermal resistances.

FIG. 5(a) and FIG. 5(b) show the temperature of the laser light sourceand the nonlinear optical component, emission wavelength of the laserlight source and phase matching wavelength of the nonlinear opticalcomponent, and wavelength mismatch error for a first comparative examplewhen (a) the wavelength mismatch error is zero at switch on and (b) thewavelength mismatch error is zero at steady state.

FIG. 6(a) and FIG. 6(b) show the temperature of the laser light sourceand the nonlinear optical component, emission wavelength of the laserlight source and phase matching wavelength of the nonlinear opticalcomponent, and wavelength mismatch error for a second comparativeexample when (a) the wavelength mismatch error is zero at switch on and(b) the wavelength mismatch error is zero at steady state.

FIG. 7(a) and FIG. 7(b) show the temperature of the laser light sourceand the nonlinear optical component, emission wavelength of the laserlight source and phase matching wavelength of the nonlinear opticalcomponent, and wavelength mismatch error for a third comparative examplewhen (a) the wavelength mismatch error is zero at switch on and (b) thewavelength mismatch error is zero at steady state.

FIG. 8(a) and FIG. 8(b) show the temperature of the laser light sourceand the nonlinear optical component, emission wavelength of the laserlight source and phase matching wavelength of the nonlinear opticalcomponent, and wavelength mismatch error for a fourth comparativeexample when (a) the wavelength mismatch error is zero at switch on and(b) the wavelength mismatch error is zero at steady state.

FIG. 9(a) and FIG. 9(b) show the temperature of the laser light sourceand the nonlinear optical component, emission wavelength of the laserlight source and phase matching wavelength of the nonlinear opticalcomponent, and wavelength mismatch error for a fifth comparative examplewhen (a) the wavelength mismatch error is zero at switch on and (b) thewavelength mismatch error is zero at steady state.

FIG. 10 shows experimental data on the dependence of temperaturetolerance and power of the output light of a laser device according tothis invention on the convergence half-angle of the source lightincident on the nonlinear optical component.

FIG. 11(a) and FIG. 11(b) show the range of available phase matchingdirections for (a) a source light with a large convergence half-angleand (b) a source light with a small convergence half-angle.

FIG. 12 shows experimental data on the dependence of temperaturetolerance of laser devices according to this invention on theconvergence half-angle of the source light incident on the nonlinearoptical component for a device with a laser light source configured toemit source light with a broad spectral linewidth and for a device witha laser light source configured to emit light with a narrow spectrallinewidth.

FIG. 13 shows experimental measurements of the position of the beam ofoutput light for source light with different wavelengths.

FIG. 14(a), FIG. 14(b), FIG. 14(c), FIG. 14(d), and FIG. 14(e) showschematic diagrams of output light propagating out of the nonlinearoptical component at three different operating conditions and theeffects of various beam stabilising optical components on the directionof the output light.

FIG. 15 shows an exemplary laser device according to an aspect of thisinvention.

FIG. 16 is a table showing suitable values for θ for Type 1 phasematching in a β-BaB₂O₄ crystal for source light with different inputwavelengths

.

FIG. 17 shows the optimum phase matched wavelength as a function ofcrystal temperature for a β-BaB₂O₄ crystal, as measured experimentally.

FIG. 18 shows an exemplary laser device according to an aspect of thisinvention which includes a beam folding optical component.

FIG. 19 shows the power of the output light from a device constructedaccording to the second example of this invention as a function ofambient temperature, as measured experimentally.

FIG. 20 shows an exemplary laser device according to an aspect of thisinvention which includes a light absorbing component to heat thenonlinear optical component.

FIG. 21 shows an exemplary laser device according to an aspect of thisinvention which includes one or more optical components which stabilisethe position and/or direction of the output light beam.

FIG. 22 shows an exemplary laser device according to an aspect of thisinvention which includes a wavelength stabilising component.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1. Device    -   2. Laser light source    -   3. Source light    -   4. Nonlinear optical component    -   5. Output light    -   6. Filter    -   7. Residual light    -   8. Housing    -   9. Output optical component    -   10. Optional optical components    -   11. Collimating lens    -   12. First cylindrical lens    -   13. Second cylindrical lens    -   20. Heat sink    -   30. First fixture point    -   31. Second fixture point    -   40. Beam folding optical component    -   50. Light absorbing component    -   51. Optional additional optical components    -   60. Optical component    -   70. Wavelength stabilising component    -   71. Incident light    -   72. Optical component    -   80. Output light beam of a first direction    -   81. Output light beam of a second direction    -   82. Output light beam of a third direction    -   90. Convex lens    -   91. Diffraction grating    -   92. Prism    -   93. Off-axis parabolic mirror    -   100. Optional thermal resistance-increasing component

DETAILED DESCRIPTION OF INVENTION

The invention provides a device for generating light, for exampleultraviolet light, which overcomes impracticalities found in previousconventional devices. In particular, the device of the present inventionis effective over a wide range of ambient temperature conditions and/orother operating conditions without the need for active temperaturecontrol of the device.

A device according to an aspect of this invention is illustrated inFIG. 1. The device 1 includes a laser light source 2 which emits sourcelight 3, including a range of wavelengths and with a first peakwavelength

_(peak, source), which is incident on a nonlinear optical component 4.The source light 3 may optionally be coupled into the nonlinear opticalcomponent by the use of one or more optical components 10. Within thenonlinear optical component 4, the source light 3 undergoes a nonlinearfrequency conversion process which generates an output light 5 at leasta portion of which has a second peak wavelength different from the firstpeak wavelength of the source light 3, λ_(peak, output)≠

_(peak, source). The nonlinear frequency conversion process may besecond harmonic generation (SHG), for whichλ_(peak, output)≈Λ_(peak, source)/2. The output light may have a peakwavelength which is in the range 200 nm≦λ_(peak, output)≦300 nm.Throughout this disclosure, wavelengths refer to the wavelength of lightmeasured when propagating in vacuum where the refractive index is equalto one.

The efficiency of a frequency conversion process in a nonlinear opticalcomponent may depend on several factors, including the wavelength of thesource light 3, the direction of the source light within the nonlinearoptical component (for example with respect to crystal axes of saidcomponent, especially if the nonlinear optical component includesoptically birefringent material), and the temperature of the nonlinearoptical component. For a particular configuration (e.g. a temperature ofthe nonlinear optical component and a direction of the source lightwithin said component), the efficiency of a frequency conversion processdepends on the wavelength of the source light, and the wavelength of thesource light for which the frequency conversion process has maximumefficiency is referred to herein as

_(PM). The efficiency of the frequency conversion process may be amaximum when the source light 3 and the output light 5 propagate withinthe nonlinear optical component with a constant phase relationship, acondition generally known as a phasematched condition.

The nonlinear optical component 4 is preferably configured such that

_(PM)≈

_(peak, source) for an initial design operating condition. For example,the nonlinear optical component is configured such that the frequencyconversion process is partially or fully phasematched for an initialdesign operating condition. This may be achieved by configuring thecomponents such that the direction of propagation of the incident sourcelight 3 inside the nonlinear optical component, with respect to thecrystal axes of the nonlinear optical component, fulfils thephasematching condition for a frequency conversion process. Thewavelength mismatch error, α, is defined herein as the differencebetween the peak wavelength of the source light and the wavelength forwhich the nonlinear frequency conversion process in the nonlinearoptical component has a maximum value (i.e. α=

_(peak, source)−

_(PM)). The efficiency of the frequency conversion process generallybecomes smaller as the absolute value of the wavelength mismatch error(i.e. |α|) increases. The maximum wavelength mismatch error for whichthe laser device is effective is defined herein as the acceptablewavelength mismatch error, ω. The laser device is effective when theabsolute value of the wavelength mismatch error is less than or equal tothe acceptable wavelength mismatch error (i.e. |α|≦ω); the laser deviceis not effective when the absolute value of the wavelength mismatcherror is greater than the acceptable wavelength mismatch error (i.e.|α|>ω). As an example, the laser device is effective when the opticalpower of the output light 5 is above a minimum acceptable power. Aspectsof the present invention provide a laser device with a small wavelengthmismatch error and/or a large acceptable wavelength mismatch error, andtherefore the laser device is unexpectedly and advantageously effectiveover a wide range of operating conditions.

Using source light which is laser light produces a frequency convertedoutput light which has laser-like properties, including narrow spectrallinewidth, high beam quality and strong linear polarisation. Theseproperties are highly desirable for certain applications, such as insensors which utilise absorption, fluorescence or scattering of thefrequency converted light for a sensing function, because laser-likelight can be better collimated than a non-laser-like light therebyachieving a longer path length and higher sensitivity. Laser-like lightcan also be focused to a smaller spot size than non-laser-like lightwhich achieves higher power densities and enables smaller, lower costdetectors to be used in said sensors.

Additional optional optical components may act on the output light 5after it has propagated out of the nonlinear optical component 4. Forexample, one or more output optical components, which may include afilter 6, may be used to reduce the amount of residual light 7 whichoverlaps with the output light 5. The residual light is source light 3which passes through the nonlinear optical component but does notundergo the frequency conversion process. One or more output opticalcomponents 9 may be used to focus, collimate or redirect the residuallight 7 or the output light 5. The elements of the device 1 may beenclosed within a housing 8.

Aspect: Laser Light Source and Nonlinear Component Configured for Smallα

In an aspect of the invention the laser light source 2 and nonlinearoptical component 4 are configured to maintain a small wavelengthmismatch error even when operating conditions for the device 1 arechanged.

The wavelength mismatch error may be kept small by configuring the laserlight source 2 and the nonlinear optical component 4 such that a changein the peak wavelength of the source light 3 with temperature,ΔΛ_(peak,source)/ΔT|_(T) _(j) , and a change with temperature of thewavelength for which the efficiency of the frequency conversion processis a maximum, ΔΛ_(PM)/ΔT|_(T) _(j) have values which are approximatelyclosely matched to one another (i.e. ΔΛ_(peak,source)/ΔT|_(T) _(j)≈ΔΛ_(PM)/ΔT|_(T) _(j) ). For example, a laser device may be operableover a range of temperatures equal to 2·T_(range) when the laser lightsource and nonlinear optical component are configured such that thefollowing equation is satisfied:

${{{{{{\Delta\Lambda}_{{peak},{source}}/\Delta}\; T}❘_{T_{j}}{{{{- {\Delta\Lambda}_{PM}}/\Delta}\; T}❘_{T_{j}}}}} < \frac{\omega}{T_{range}}},$where T_(j) is an ambient temperature (for example the temperature of asurface of an apparatus that the device 1 is attached to). In apreferred example,|ΔΛ_(peak,source) /ΔT| _(T) _(j) −ΔΛ_(PM) /ΔT| _(T) _(j) |<0.05 nmK⁻¹and in a yet more preferred example,|ΔΛ_(peak,source) /ΔT| _(T) _(j) −ΔΛ_(PM) /ΔT| _(T) _(j) |<0.03 nmK⁻¹

This aspect of the invention provides a device which is effective over asignificantly wider range of operating conditions, compared to a devicewhich does not fulfil this criterion, for the following changes inoperating conditions:

-   -   1. Changes in ambient temperature.    -   2. Changes in the electrical current and/or voltage supplied to        the laser light source 2.    -   3. Changes from continuous wave operation to pulsed operation or        vice versa, or a change in pulse length and/or duty cycle during        pulsed operation.

The range of operating conditions over which the device is effective isreferred to herein as the operable range, and is measured with respectto a particular operating condition, such as ambient temperature. Inthis case the operable range may be defined as the range of ambienttemperatures over which the power of the output light 5 remains above afraction (β) of the maximum power of the output light within the range,where other parameters (for example the electrical current supplied tothe laser light source 2) are unchanged. For example, β may be greaterthan 0.1, preferably β is greater than 0.5. The acceptable value of βdepends on the application for the device 1. Alternatively, the operablerange with respect to ambient temperature may be defined as the range ofambient temperatures over which the power of the output light exceeds aparticular value while other operating parameters remain within a targetrange. For example, the electrical current supplied to the laser lightsource remains below a maximum allowable value, such that the device mayprovide output light with power of at least a minimum power throughoutthe operating range of ambient temperatures without exceeding themaximum allowable electrical current.

As stated previously, the nonlinear optical component is configured suchthat

_(PM)≈

_(peak, source) for an initial design operating condition including, forexample, an ambient temperature.

_(peak, source) and

_(PM) may be dependent on the temperature of the respective components.Therefore,

_(PM)≈

_(peak, source) may only be satisfied for the design operatingcondition. The temperatures of the laser light source 2 and nonlinearoptical component 4 may move away from those of the design operatingcondition due to one or more of the following:

-   -   1. A change in ambient temperature in the vicinity of the        components. An increase in ambient temperature will lead to an        increase in component temperatures while a decrease in ambient        temperature will lead to a decrease in component temperatures.    -   2. A change in electrical current and/or voltage supplied to the        laser light source 2. The electrical power supplied to the laser        light source will not be converted to light with 100%        efficiency. The remaining power which is not converted to light        is instead converted to heat. In addition to directly heating        the laser light source itself, this heat may dissipate out from        the laser light source. For example, the heat may be transferred        to a mount containing the laser light source or to the air        around the laser light source. Some of this heat may be        transferred to the nonlinear optical component 4 via conduction        between the mount of the laser light source and the nonlinear        optical component, via convection in the air between the laser        light source and the nonlinear optical component or via some        other means. This results in an increase in temperature of the        nonlinear optical component. If the electrical current and/or        voltage supplied to the laser light source decreases then the        amount of heat generated will decrease and the temperature of        the components will decrease. Similarly, if the electrical        current and/or voltage increases then the amount of heat        generated will increase and the temperature of the components        will increase. This changing condition includes switch-on of the        device wherein the electrical current supplied to the laser        light source increases from zero.    -   3. A change in operating mode of the laser light source from        continuous wave to pulsed (or vice versa) or a change in pulse        length and/or duty cycle of pulsed operation. Relative to a        continuous wave injection current, when the device is operated        in a pulsed condition for the same injection current the amount        of heat generated by the laser light source decreases because        the average electrical power supplied to the laser light source        is reduced. Similarly for a change in pulse length or duty        cycle, the amount of heat generated by the laser light source        may change. A reduction in the amount of heat generated leads to        lower component temperatures.

As the component temperatures moves away from those of the designoperating condition,

_(peak, source) and

_(PM) may no longer be approximately equal to one another (i.e. theabsolute value of the wavelength mismatch error (|α|) becomes greaterthan zero). This results in a decrease in the efficiency of thefrequency conversion process, with a larger absolute value of thewavelength mismatch error generally resulting in a greater decrease inthe nonlinear conversion efficiency. This leads to a reduced power ofthe output light 5. The present inventors have found that this behaviourmay severely limit the range of operating conditions for which thedevice 1 is effective, and thereby reduce the practical application ofthe device. In a first example, the power of the output light may varysignificantly as one or more of the operating conditions 1, 2 and 3above are changed, such that the device may only be practically used fora narrow range of said conditions. The limitation on the mode ofoperation of the device (operating condition 3 above) is presented hereby the inventors for the first time and is a problem which has notpreviously been appreciated in the prior art.

Through configuring the laser light source and the nonlinear opticalcomponent such that|ΔΛ_(peak,source) /ΔT| _(T=T) _(op) −ΔΛ_(PM) /ΔT| _(T=T) _(op) |<0.05nmK⁻¹

where T_(op) is an ambient temperature at which the device 1 is requiredto operate, the present inventors find that the device is effective overa wide range of operating conditions.

It is additionally advantageous for the laser light source 2 andnonlinear optical component 4 to be configured such that|ΔΛ_(peak,source) /ΔT| _(T) _(min) _(≦T≦T) _(max) −ΔΛ_(PM) /ΔT| _(T)_(min) _(≦T≦T) _(max) |<0.05 nmK⁻¹over the desired operable range of ambient temperature between a minimumtemperature T_(min) and a maximum temperature T_(max).

The schematic diagram in FIGS. 2(a)-2(b) illustrate the advantage ofthis aspect of the invention. The plot in FIG. 2(a) shows an exemplarydependence of

_(peak, source) and

_(PM) on temperature according to this aspect. In this case ΔΛ_(PM)/ΔTand ΔΛ_(peak,source)/ΔT each have a constant value throughout thetemperature range. The device 1 is configured to have a wavelengthmismatch error approximately equal to zero for an initial designcondition, when the laser light source 2 and nonlinear optical component4 are at the same temperature. Wavelength mismatch error (α) remainssmall when the laser light source and the nonlinear optical componenthave similar temperature, throughout the temperature range shown in theplot. Furthermore, in an exemplary operating condition illustrated inFIG. 2(a) for which the laser light source has a temperature T₂ and thenonlinear optical component has a different temperature T₁ thewavelength mismatch error remains small, In comparison, the plot in FIG.2(b) shows the dependence of

_(peak, source) and

_(PM) on temperature when the laser light source and nonlinear opticalcomponent are not configured according to this aspect of the invention.The wavelength mismatch error becomes large when the temperature ofeither component changes from the design condition.

A common solution found in conventional devices to achieve a largeoperable range is to use active temperature control which maintainseither or both the laser light source 2 and nonlinear optical component4 at a constant temperature or within a target temperature range,regardless of the ambient temperature. This requires adding one or morecomponents to the laser light source and/or nonlinear optical component.The one or more components determine the temperature of the laser lightsource and nonlinear optical component and move heat into or out of thedevice based on a feedback loop to minimise the difference between thedetermined temperature and a target temperature of the laser lightsource and nonlinear optical component (for example the temperature ofthe design operating condition). While this approach is capable ofproducing a device with a wide operable range (for example with respectto ambient temperature, or with respect to duty cycle of pulsedoperation), there are several drawbacks: (i) Active temperature controlincreases the complexity of the device by requiring additionalcomponents and control circuitry. This increase in complexity inevitablyleads to an increase in manufacturing costs and increases the size ofthe device. (ii) Operating the one or more components used for activetemperature control will also consume energy, resulting in increasedrunning costs for the end user. (iii) If it is not acceptable to operatethe active temperature control continuously (for example to minimiseenergy consumption), then there is a delay between when the activetemperature control is activated and when the output light may begenerated, while the component(s) in the device reach the requiredtemperature or temperatures. The present invention overcomes thesedisadvantages by providing wide operating range with respect to ambienttemperature.

Aspect: Stabilised Laser Light Source Wavelength for Small α

In another aspect of the invention, one or more optional opticalcomponents may be configured to stabilise the emission wavelength of thelaser light source 2 with respect to variations in temperature, changesto the electrical current supplied to the laser light source and/orchanges in the operating mode of the laser light source. This mayfurther reduce the wavelength mismatch error caused by changes inoperating conditions. The optical components reduce the magnitude ofΔΛ_(peak,source)/ΔT to a new value ΔΛ′_(peak,source)/ΔT such that|ΔΛ′_(peak,source)/ΔT|<|ΔΛ_(peak,source)/ΔT|, as illustrated in FIG. 3.This is particularly advantageous in cases where ΔΛ′_(peak,source)/ΔT iscloser in value to ΔΛ_(PM)/ΔT than ΔΛ_(peak,source)/ΔT is to ΔΛ_(PM)/ΔT.The wavelength mismatch error (α) may then be reduced and the device iseffective over a wider operable range (e.g. with respect to ambienttemperature) than without the additional optical components. This isadvantageous in situations when it is desirable to use a particularcombination of laser light source and nonlinear optical component 4 butwhere either sgn(ΔΛ_(peak,source)/ΔT)≠sgn(ΔΛ_(PM)/ΔT) orΔΛ_(peak,source)/ΔT>2. ΔΛ_(PM)/ΔT. The nomenclature “sgn(x)” refers tothe mathematical sign of the parameter x (i.e. positive or negative).For example, a device designed to generate a particular outputwavelength λ_(peak, output) may mean that a combination of laser lightsource and nonlinear optical component with ΔΛ_(peak,source)/ΔT≦2.ΔΛ_(PM)/ΔT suffers from increased cost, increased complexity or simplydoes not exist.

Aspect: Housing with Thermal Contact for Small α.

Generally, an aspect of the invention is a laser device. In exemplaryembodiments, the laser device includes a light source configured to emita source light having a first peak wavelength, and a nonlinear opticalcomponent configured to perform a frequency conversion process thatconverts at least a portion of the source light into output light havinga second peak wavelength different from the first peak wavelength. Astabilization component is configured to minimize a mismatch errorconstituting a difference between the first peak wavelength of thesource light and a wavelength for which the frequency conversion processin the nonlinear optical component has a maximum value. Thestabilization may include a housing that is thermally conductive betweenthe light source and the nonlinear optical component to minimize atemperature difference between the light source and the nonlinearoptical component.

Referring more particularly to FIG. 1, in another aspect of theinvention, the laser light source 2 and nonlinear optical component 4are both attached to a housing 8. The housing is configured to provide alarge operable range for the device 1 with respect to changing operatingconditions. For example, the housing is configured to provide anadvantageously small wavelength mismatch error when there are changes inambient temperature, changes to the electrical current supplied to thelaser light source and/or changes in the operating mode of the laserlight source. In one example, the housing is configured to have thermalconduction properties which provide a small wavelength mismatch error.In particular, the thermal conduction properties of the housing may beconfigured to exploit the condition according to an earlier aspect ofthe invention (specifically that|ΔΛ_(peak) /ΔT| _(T=T) _(op) −ΔΛ_(PM) /ΔT| _(T=T) _(op) |<0.05 nmK⁻¹)such that said thermal conduction properties provide an advantageouslysmall difference between the temperature of the laser light source andthe temperature of the nonlinear optical component for a wide range ofoperating conditions.

The housing is configured to provide a good thermal contact between thelaser light source 2 and the nonlinear optical component 4, such that achange in temperature of the laser light source causes a change in thetemperature of the nonlinear optical component. When the laser lightsource and the nonlinear optical component are in good thermal contactwith each other, the difference between the temperature of the laserlight source and the temperature of the nonlinear optical componentduring operation of the laser is smaller than said temperaturedifference if the laser light source and nonlinear optical componentwere thermally isolated from each other. The aforementioned temperaturedifference may be lower both during steady-state operation, and duringnon-steady-state operation. Furthermore, when the laser light source andthe nonlinear optical component are in good thermal contact with eachother, the difference between the temperature of the laser light sourceand the temperature of the nonlinear optical component is small even ifthere is a change in the ambient temperature, or a temperature gradientin the external environment.

The thermal properties of housing 8 may determine the relationshipbetween the temperature of the laser light source 2 and the temperatureof the nonlinear optical component 4. The thermal properties of thehousing may be configured advantageously to:

-   -   1. Reduce the variation in power of the output light from the        device for a constant electrical injection current while the        device is not in a steady state thermal condition.    -   2. Reduce the variation in electrical injection current which is        necessary to maintain a constant power of the output light while        the device is not in a steady state thermal condition.    -   3. Decrease the time taken for the device to reach to a steady        state thermal condition.

Conditions when the device is not in a steady state thermal conditioninclude:

-   -   1. After the laser light source 2 has been switched on and        before the device 1 reaches a steady thermal state at which the        temperature of components does not change significantly with        time. This may be considered as a “warm-up time”.    -   2. When the ambient temperature is not constant with time.

For example, the thermal properties of the housing 8 may be configuredto provide good thermal contact between the laser light source and thenonlinear optical component using one or more of the following:

-   -   1. The distance between the laser light source and nonlinear        optical component is less than 100 mm, preferably less than 50        mm, and most preferably less than 30 mm.    -   2. The thermal conductivity of material in the housing which        links the laser light source and the nonlinear optical component        is at least 10 W·m⁻¹·K⁻¹, and preferably at least 100 W·m⁻¹·K⁻¹.    -   3. The heat capacity of the housing between the laser light        source and the nonlinear optical component is less than 500        J·K⁻¹, preferably less than 200 J·K⁻¹, and most preferably less        than 50 J·K⁻¹.

Furthermore, the heat capacity of the nonlinear optical component isless than 0.1 J·K⁻¹, advantageously facilitating rapid heating orcooling of said component in response to changes in the temperature ofthe housing 8.

Suitable values for the above parameters may depend on the configurationof the laser light source, the nonlinear optical component and therequired operating range of the laser device, as will be explained inexamples later in this disclosure.

When the thermal properties of the housing 8 are configured to providegood thermal contact between the laser light source and the nonlinearoptical component, the difference in temperature between said componentsduring operation of the device is preferably never more than 40° C., andmore preferably never more than 10° C. These maximum temperaturedifferences ensure sufficiently small wavelength mismatch errors foreffective operation of the device.

The good thermal contact between the laser light source 2 and thenonlinear optical component 4 may be described by the value of theabsolute thermal resistance between said components. The absolutethermal resistance between two components is defined herein as thetemperature difference between said components which is observed for aheat flow of 1 watt between said components (i.e. units of K·W⁻¹). Asillustrated schematically in FIG. 4, the heat flow between the laserlight source and the nonlinear optical component is primarily determinedby the absolute thermal resistance between the laser light source andthe nonlinear optical component, R_(1,2), the absolute thermalresistance between the laser light source and a heat sink 20, R_(1,3),and the absolute thermal resistance between the nonlinear opticalcomponent and the heat sink, R_(2,3). The heat sink is a body with largethermal mass such that its temperature does not change significantly inresponse to the heat generated by the laser device. For example, theheat sink may be the air surrounding the laser device, or the airsurrounding cooling fins which are attached to the laser device. Thethermal resistance between the laser light source and other components(i.e. R_(1,2) and R_(1,3)) is preferably defined from the externalpackage of the laser light source. For example, if the laser lightsource is a laser diode mounted inside a metal can package, the thermalresistance is preferably defined from the metal can package.

The housing 8 may be advantageously configured to provide values of oneor more of R_(1,2), R_(1,3) and R_(2,3) such that:

-   -   i) R_(1,2) is small. This facilitates transfer of heat between        the laser light source and the nonlinear optical component so        that a change in temperature of one component quickly leads to a        change in temperature of the other component, thereby reducing a        wavelength mismatch error. R_(1,2) is preferably less than 100        K·W⁻¹, more preferably less than 10 K·W⁻¹, and most preferably        less than 1 K·W⁻¹.    -   ii) R_(1,2)<R_(1,3). This prevents preferential flow of heat        from the laser light source directly to the heatsink, without        heating of the nonlinear optical component, thereby reducing a        wavelength mismatch error.    -   iii) R_(1,2)+R_(2,3)<R_(1,3). This facilitates flow of heat from        the laser light source to the heatsink along a thermal path via        the nonlinear optical component, providing rapid transfer of        heat from the laser light source to the nonlinear optical        component and thereby reducing a wavelength mismatch error.    -   iv) R_(1,3), R_(2,3) and R_(3,3) are all small. R_(3,3) is        defined as the thermal resistance within the heatsink. This        facilitates flow of heat from the laser light source to        nonlinear optical component and thereby reduces a wavelength        mismatch error. R_(1,3), R_(2,3) and R_(3,3) are preferably all        less than 10 K·W⁻¹.

Three comparative examples demonstrate the advantages of this aspect ofthe invention. The plots in FIGS. 5(a), 6(a) and 7(a) show schematicallythe temperature of the laser light source 2, the temperature of thenonlinear optical component 4 and the wavelength mismatch error (α) inthe time period after the laser light source is switched on.

For the first comparative example (FIG. 5(a)), the laser light source 2and the nonlinear optical component 4 are not attached to a housing 8,and there is negligible thermal contact between the two components(R_(1,2) is very large; for example at least 1000 K·W⁻¹). Initially thelaser light source and the nonlinear optical component are both at asimilar temperature, for example the ambient temperature. The laserlight source temperature increases when the laser light source isswitched on and its temperature tends towards a steady state value whichis reached at time t_(ss)(laser). By this time the rate of thermalenergy generated in the laser light source equals the rate of thermalenergy dissipation to the heat sink 20. Meanwhile, the temperature ofthe nonlinear optical component is unchanged from its initial value. Asa consequence, the wavelength mismatch error changes significantlyduring the period between the time the laser light source is switched onand the time that the device reaches a steady thermal state. Thisexample applies if the components are mounted onto an optical bench in alaboratory-based laser. For example, if R_(1,2)=1000 K·W⁻¹,R_(2,3)=R_(1,3)=10 K·W⁻¹, the heat sink temperature is 25° C., in steadystate the laser light source temperature is approximately 65° C., thenonlinear optical component temperature is approximately 25° C.,resulting in a large difference in temperature of 40° C. and causing alarge wavelength mismatch error.

For the second comparative example (FIG. 6(a)), the laser light source 2and the nonlinear optical component 4 are both attached to a housing 8and have good thermal contact, for example where the thermal resistanceof the housing between the two components (R_(1,2)) is approximately 10K·W⁻¹. The laser light source temperature increases when the laser lightsource is switched on and its temperature tends towards a steady statevalue which is reached at time t_(ss)(laser). A flow of heat from thelaser light source to the nonlinear optical component causes thetemperature of the nonlinear optical component to rise. Owing to thesmaller thermal resistance (R_(1,2)) relative to the first example, thetemperature of the nonlinear optical component increases more quicklyand, reaches steady state at time t_(ss)(nlc), lower than the steadystate temperature of the laser light source. The wavelength mismatcherror changes less during the period than for the first comparativeexample. For example, if R_(1,2)=10 K·W⁻¹, R_(2,3)=R_(1,3)=10 K·W⁻¹, theheat sink temperature is 25° C., in steady state the laser light sourcetemperature is approximately 52° C., the nonlinear optical componenttemperature is approximately 38° C., resulting in a small difference intemperature of 14° C. and consequently an advantageously smallwavelength mismatch error.

For the third comparative example (FIG. 7(a)), the laser light source 2and the nonlinear optical component 4 are both attached to a housing 8and have very good thermal contact, for example where the thermalresistance of the housing between the two components (R_(1,2)) is lessthan 1 K·W⁻¹. The laser light source temperature increases when thelaser light source is switched on and its temperature tends towards asteady state value which is reached at time t_(ss)(laser). A flow ofheat from the laser light source to the nonlinear optical componentcauses the temperature of the nonlinear optical component to rise. Owingto the small thermal resistance (R_(1,2)) relative to the first andsecond examples, the temperature of the nonlinear optical componentrises very quickly and reaches a steady state (at time t_(ss)(nlc))which is similar to the steady state temperature of the laser lightsource. The wavelength mismatch error changes significantly less duringthe period than for the first comparative example. For example, ifR_(1,2)=1 K·W⁻¹, R_(2,3)=R_(1,3)=10 K·W⁻¹, the heat sink temperature is25° C., in steady state the laser light source temperature isapproximately 46° C., the nonlinear optical component temperature isapproximately 44° C., resulting in a very small difference intemperature of 2° C. and consequently an advantageously small wavelengthmismatch error.

In the above first through third comparative examples, the laser lightsource and nonlinear optical component were configured such that

_(PM) and

_(peak) were approximately equal when the laser light source wasswitched off (e.g. both components at ambient temperature).Alternatively, the laser light source and the nonlinear opticalcomponent may be configured such that

_(PM) at t_(ss)(nlc) is equal to

_(peak) at t_(ss)(laser). This reduces the wavelength mismatch errorduring steady state operation but results in the wavelength mismatcherror being non-zero when the laser light source is switched on, asshown in FIGS. 5(b), 6(b) and 7(b). This may lead to a period afterswitch on of the device where the wavelength mismatch error isunacceptably large, thus requiring that the device be allowed to “warmup” before it may be used. It can be seen for the second and thirdcomparative examples that configuring the nonlinear optical component inthis way advantageously reduces the maximum value of |α| compared to adevice where the nonlinear optical component is configured such that α=0when the laser light source is switched on.

The addition of a housing 8 with optimised thermal properties brings theadvantage of a device which has the desirable properties of a reducedvariation in output power when the device is not in a steady statecondition and a reduced time taken to reach a steady state conditionfrom a non-steady state condition.

Aspect: Residual Laser Light Heats the Nonlinear Optical Component forSmall α

In another aspect of the invention, the laser device is configured sothat some or all the source light 3 which passes through the nonlinearoptical component 4 but does not undergo the frequency conversionprocess (also referred to as the residual light 7) is utilised to heatthe nonlinear optical component. This may provide an advantageouslysmall wavelength mismatch error when there are changes to the electricalcurrent supplied to the laser light source 2 and/or changes in theoperating mode of the laser light source. Fourth and fifth comparativeexamples demonstrate the advantage of this aspect of the invention (madewith reference to the first, second and third comparative examplesdescribed previously).

For the fourth comparative example (FIG. 8(a)), the laser light source 2and the nonlinear optical component 4 are both attached to a housing 8,optionally the thermal resistance of the housing between the twocomponents (R_(1,2)) is small, and the housing is configured so that theresidual light 7 is utilised as an additional source of heat for thenonlinear optical component. The some or all of the residual light isdirected—for example using one or more mirrors—such that it is incidenton an absorbing component which is in thermal contact with the nonlinearoptical component. Some or all of the residual light which is incidenton the absorbing component is absorbed at the absorbing component suchthat the absorbed light causes the temperature of the absorbingcomponent to increase and, in turn, this causes the temperature of thenonlinear optical component to increase. The laser light sourcetemperature increases when the laser light source is switched on and itstemperature tends towards a steady state value which is reached at timet_(ss)(laser). Heating of the absorbing component, which is caused byabsorption of residual light at the absorbing component, and optionallyalso a flow of heat from the laser light source to the nonlinear opticalcomponent, cause the temperature of the nonlinear optical component torise. Owing to this heating effect, the temperature of the nonlinearoptical component rises quickly and reaches a steady state (at timet_(ss)(nlc)) which is similar to the steady state temperature of thelaser light source. The wavelength mismatch error changes significantlyless during the period than for the first comparative example.

The power of the residual light 7 which is incident on the absorbingcomponent may be configured to provide advantageously favourable changein temperature of the nonlinear optical component when the laser lightsource is switched on. For example, the fraction of the residual lightwhich is directed onto the absorbing component may be configured toprovide an advantageously favourable change in temperature of thenonlinear optical component. The thermal resistance between thenonlinear optical component and the heat sink and/or the laser lightsource may be configured to provide a preferred steady-state temperatureof the nonlinear optical component.

For the fifth comparative example, (FIG. 9(a)), the power of theresidual light 7 incident on the absorbing component is configured sothat the steady-state temperature of the nonlinear optical component 4,when the laser light source 2 is switched on, is higher than thetemperature of the laser light source. The laser light sourcetemperature increases when the laser light source is switched on and itstemperature tends towards a steady state value which is reached at timet_(ss)(laser). Heating of the absorbing component is caused byabsorption of residual light at the absorbing component. Owing to thisheating effect, the temperature of the nonlinear optical component risesquickly and reaches a steady state (at time t_(ss)(nlc)) which is higherthan the steady state temperature of the laser light source. Thewavelength mismatch error changes significantly less during the periodthan for the first comparative example. It may be advantageous for thesteady-state temperature of the nonlinear optical component to be higherthan the steady-state temperature of the laser light source ifΔΛ_(peak,source)/ΔT|_(T=T) _(j) >ΔΛ_(PM)/ΔT|_(T=T) _(j) .

In the same manner as for the first through third comparative examples,the nonlinear optical component may be configured such that

_(PM) at t_(ss) (nlc) is equal to

_(peak, source) at t_(ss)(laser). The effect of this on the fourth andfifth comparative examples is shown in FIGS. 8(b) and 9(b).

For this aspect of the invention, where the residual light 7 is utilisedas an additional source of heat for the nonlinear optical component 4,it is especially advantageous if the heat capacity of the nonlinearoptical component is less than 0.1 J·K⁻¹, advantageously facilitatingrapid heating or cooling of said component in response to changes in thetemperature of the housing 8.

Aspect: Focusing of Laser Light for Large ω

In another aspect of this invention the laser device 1 is configured sothat the acceptable wavelength mismatch error, ω, is large and thereforethe laser device has a large operable range with respect to changingoperating conditions. According to this aspect of the invention, thelaser device is configured so that the source light 3 incident on thenonlinear optical component 4 has a convergence angle which provides alarge acceptable wavelength mismatch error. The focusing condition willbe described throughout this disclosure as the far-field convergencehalf-angle in air of the laser light which is incident on the nonlinearoptical component. For brevity this may also be referred to as theconvergence half-angle.

The present inventors have determined that there is an advantageousincrease in the acceptable wavelength mismatch error when the device 1is configured such that a convergence half-angle of the source light 3is different from a value which provides the maximum frequencyconversion efficiency or maximum output power (i.e. the convergencehalf-angle which would conventionally be used to provide a frequencyconverted laser device to provide maximum output power). In particular,there is an advantageous increase in the acceptable wavelength mismatcherror when a convergence half-angle is significantly larger than thevalue which provides the maximum frequency conversion efficiency. Fornonlinear optical frequency conversion in which phase matching isprovided through birefringent phase matching, there is a direction inthe nonlinear optical component for which rotation about said directionprovides the largest change in phase matched wavelength of input lightwith respect to a change in angle of rotation about said direction. Thisdirection is referred to herein as the phase matching rotation axis. Itis particularly advantageous for increasing the acceptable wavelengthmismatch error of the device if the convergence half-angle of the sourcelight measured in the plane which is perpendicular to the phase matchingrotation axis of the nonlinear optical component is larger than thevalue which provides the maximum frequency conversion efficiency. Datain FIG. 10 illustrates the advantages. This data, which is explained inmore detail in Example 1, demonstrates a significant advantageousincrease in operable range of a laser when a convergence half-angle isincreased away from the value which provides maximum frequencyconversion efficiency.

The optimum convergence half-angle for the laser light incident on thenonlinear optical component to provide a device which maximises thepower of frequency-converted (e.g. SHG) output light may be calculatedaccording to the prior art. For example, the method of Freegarde et al.[Journal of the Optical Society of America B 14, 2010 (1997)], or asdisclosed in US patent application number 2015/0177593A1, may be used.If a larger convergence half-angle is used according to this aspect ofthe invention, for example by reducing the focal length of the lens usedto focus the laser beam into the nonlinear optical component, the beamcontains a larger spread of directions through the nonlinear opticalcomponent. This may act to broaden the range of wavelengths which areable to undergo phase matched frequency conversion, as illustratedschematically in FIGS. 11(a) and 11(b) with FIG. 11(a) corresponding toa broader range of wavelengths as compared to FIG. 11(b). The effect ofthis is to reduce the maximum achievable frequency conversionefficiency, but to increase the acceptable wavelength mismatch error ofthe device, thereby reducing the effect of changes in operatingconditions and providing a laser device with a larger operable range.

The present inventors have determined that without this aspect of theinvention, which provides a large acceptable wavelength mismatch error,a device may exhibit poor operable range with respect to changes inoperating conditions including changes in ambient temperature andchanges between pulsed operation and continuous wave operation. Asolid-state laser light source (e.g. a semiconductor laser diode) mayexhibit a significant shift in peak emission wavelength,

_(peak, source), when changing from operating in continuous wave mode tooperating in pulsed mode or vice-versa. This results in a device 1,which is optimised to operate in continuous wave mode, havingunexpectedly poor output power if it is operated in pulsed mode, or viceversa, because of the increase wavelength mismatch error which isintroduced when the operation mode is changed. During continuousoperation more heat is generated leading to the solid-state laser lightsource reaching a higher temperature when compared with pulsedoperation. In addition, during pulsed operation there is a period whenthe laser light source is off. During this period heat is dissipatedaway from the laser light source, allowing its temperature to return toambient temperature before the next laser pulse. This aspect of theinvention, which increases the acceptable wavelength mismatch error, maybe additionally advantageous when a device is desired which is capableof being operated in both continuous wave and pulsed operation modesbecause a larger acceptable wavelength mismatch error gives animprovement in output power stability when the mode of operation of thedevice is changed. Note that in this case the improved stability isobtained between the continuous wave power and the pulsed power obtainedwhile the laser is on, rather than the average power over the wholepulse period. The need for a single device rather than two separatedevices is of benefit to the manufacturer (simplified manufacturing;reducing costs) and to the end user, especially for applications whichrequire both continuous wave and pulsed devices (one device requiredrather than two; reduced expenditure).

Furthermore, by increasing the acceptable wavelength mismatch error, thetolerance on the orientation of the nonlinear optical component 4 duringassembly is relaxed. This has the advantageous effect of simplifying themanufacturing process of the device, thereby reducing the cost of thedevice.

Aspect: Broad Range of Wavelengths in Source Light for Large ω

In another aspect of the invention, the laser device 1 includes a laserlight source 2 which is configured to emit laser light with a broadspectral linewidth. This provides a laser device with a large acceptablewavelength mismatch error, ω, and therefore the laser device has a largeoperable range with respect to changing operating conditions.

Whereas for previous aspects only the peak wavelength,

_(peak, source), has been discussed, the intensity at each wavelength ofthe spectrum of the source light 3, I(

), must now be considered. The spectral linewidth of the source light isdefined herein as the difference between an upper wavelength and a lowerwavelength, where the lower wavelength is defined such that the fractionof the power of the source light with wavelength less than the lowerwavelength is 5% of the total power of the source light, and the upperwavelength is defined such that the fraction of the power of the sourcelight with wavelength greater than the upper wavelength is 5% of thetotal power of the source light.

According to the prior art, it is preferable to use laser light with anarrow spectral linewidth (e.g. less than 0.1 nm) because this providesthe highest frequency conversion efficiency, owing to the nonlineardependence of frequency-conversion efficiency on the power of the laserlight incident on the nonlinear optical component. For example, in thecase of SHG, the power of frequency-doubled output light with wavelengthλ₁ is approximately proportional to the second order of the power of theinput light with wavelength

₁=2λ₁. Therefore, to obtain a high power of the output light, it isadvantageous for the power of the input light to be distributed over avery small range of wavelengths, rather than the same power to bedistributed over a broad range of wavelengths.

However, the present inventors have determined that, in combination withother aspects of the invention, an advantageous increase in operablerange for the laser device may be obtained if the laser light source 2is configured to have a broad spectral linewidth, such as for example, aspectral linewidth of at least 0.5 nm, and preferably at least 1 nm.

The output power of a device 1 using a laser light source configured toemit a narrow spectral linewidth may be strongly affected by a smallchange in the operating conditions which cause either

_(peak, source) or

_(PM) to change. However, the output power of a device 1 using a laserlight source configured to emit a broad spectral linewidth is found tohave much larger operable range with respect to changing operatingconditions which cause a change to

_(peak, source) or

_(PM). A first set of devices 1 were manufactured, each including alaser light source 2 emitting source light 3 with peak wavelength

_(peak, source)≈442 nm and a nonlinear optical component 4 including aβ-BaB₂O₄ crystal configured to provide Type 1 SHG of the source light.Three different devices within the first set of devices had lensesconfigured to act on the source light 3 so that the source light 3incident on the nonlinear optical component had three differentconvergence half angles (measured in the plane perpendicular to thephase matching axis of the β-BaB₂O₄ crystal), as indicated by thehorizontal axis in FIG. 12. In the three devices in the first set, thelaser light source 2 was configured so that the spectral linewidth ofthe source light was ≈0.14 nm, referred to herein as “narrow linewidth”.A second set of three devices was manufactured, the same as the firstset except that the laser light source was configured so that thespectral linewidth of the source light was ≈1.9 nm, referred to hereinas “broad linewidth”. FIG. 12 compares the experimentally measuredoperable range with respect to temperature of the nonlinear opticalcomponent 4 for devices in the first and second sets. The devices in thesecond set (“broad linewidth”) have larger operable ranges than devicesin the first set (“narrow linewidth”) with equivalent convergencehalf-angles of the source light. Here the operable range has beendefined as the temperature range of the nonlinear optical component overwhich the device delivers power of the output light more than 50% of thepower of the output light at the design condition (i.e. β>0.5), alsoknown as the full width at half maximum (FWHM). By configuring the laserlight source to have a broad spectral linewidth a device mayadvantageously be produced which has a smaller decrease in power of theoutput light as operating conditions change away from the designconditions.

Furthermore, by combination of the convergence half-angle larger thanoptimum to obtain highest efficiency frequency conversion for a singlewavelength (the previous aspect of the invention), with a broad spectrallinewidth of the laser light, an advantageous increase in operable rangemay be obtained with smaller reduction in efficiency of frequencyconversion than expected for application of either of the two aspectsindividually.

Aspect: Stabilised Direction for Output Light

In another aspect of the invention one or more beam stabilising opticalcomponents is optionally included in the device 1, where a beamstabilising optical component acts on the output light 5 exiting thenonlinear optical component 4 such that at least one of the directionand position of the output light varies less with a change in operatingcondition than without said beam stabilising optical components.

The present inventors have determined that during operation of a device1 according to the other aspects of the invention, a change in operatingconditions may cause a change in the propagation direction of the outputlight 5 exiting the nonlinear optical component 4. This change in thepropagation direction of the output light becomes significant when thedevice 1 has a large operable range, as enabled by other aspects of thisinvention.

A device 1 was manufactured including a laser light source 2 emittingsource light 3 with peak wavelength

_(peak, source)≈449 nm and a nonlinear optical component 4 including aβ-BaB₂O₄ crystal configured to provide Type 1 SHG of the source light.Lenses were configured so that source light 3 was converging in bothplanes of the beam as it was incident on the nonlinear opticalcomponent, and the source light formed a waist within the β-BaB₂O₄crystal. The output light 5 propagated out from the nonlinear opticalcomponent, the output light 5 was filtered to remove residual light 7,and the output light 5 was then incident on a CCD beam profiler. The CCDbeam profiler recorded the intensity distribution of the output light 5.The distance between the nonlinear optical component and the CCD beamprofiler was ≈8 cm. FIG. 13 displays experimental data showing theintensity distribution of the output light 5 when the laser light source2 was configured to emit source light with peak wavelength of 447.9 nm,448.9 nm and 449.6 nm (no other changes were made to the device; forexample the orientation and position of the nonlinear optical componentwere not changed). The centre of the output light beam in the horizontaland vertical directions is indicated by dotted white lines. Thedisplacement of the centre of the output light beam at the CCDcorresponds to a change in the direction of the output light propagatingout from the nonlinear optical component when

_(peak, source) was changed and with constant

_(PM). This dependence of propagation direction on operating conditionsis due to a change in the wavelength mismatch error (caused by a changein operating conditions) causing the nonlinear optical process to takeplace along different directions in the nonlinear optical component. Forexample, if the wavelength of the source light emitted by the laserlight source changes, due to a change in operating conditions, thedirection of the output light may change.

This has the previously unappreciated effect of causing the direction ofthe output light to undesirably depend on the operating conditions ofthe device. This effect is observed now owing to the advantageousincreases in operable range of the laser device provided by otheraspects of the invention. A change in the direction of the output lightwith operating conditions is disadvantageous for many applications.

One or more beam stabilising components can reduce or eliminate thechange in direction of the output light 5, such change in directiongenerally being shown in FIG. 14(a). Example beam stabilising componentsare illustrated in FIGS. 14(b)-14(e) and may be refractive, diffractive,dispersive or reflective. Beam stabilising components may include anycombination of a lens 90 located a distance from the centre of thenonlinear optical component 4, measured along the propagation directionof the output light, approximately equal to the effective focal lengthof said lens, a reflective or transmissive diffraction grating 91, aprism 92, and a mirror such as an off-axis parabolic mirror 93. Thelight beam which propagates out from the nonlinear optical componentpropagates in a direction which varies depending on the operatingcondition of the device 1. For example, as shown in FIG. 14(a) at afirst operating condition the light may propagate along a firstdirection 80, at a second operating condition the light may propagatealong a second direction 81 while at a third operating condition thelight may propagate along a third direction 82. As is shown in FIGS.14(b)-(e), the difference between the directions of propagation forlights at two different operating conditions is reduced after the lightshave interacted with the beam stabilising components. FIG. 14(a) showsthe variation in the direction of output light without a beamstabilising component. FIG. 14(b) shows the variation in the directionof the output light when a convex lens 90 is used as the beamstabilising component. FIG. 14(c) shows the variation in the directionof output light when a reflective diffraction grating 91 is used as abeam stabilising component. FIG. 14(d) shows the variation in thedirection of output light when a prism 92 is used as a beam stabilisingcomponent. FIG. 14(e) shows the variation in the direction of outputlight when an off-axis parabolic curved mirror 93 is used as beamstabilising component. For the examples in FIG. 14(b) and FIG. 14(e) theshape of the optical component (lens or mirror) provides a differentdeflection in the direction of the light depending on the direction ofthe light incident on the optical component to reduce the variation inthe final direction of the output light. For the examples in FIG. 14(c)and FIG. 14(d), the dependence of the direction of the output light onthe wavelength of the output light is combined with an optical component(grating or prism) which provides a different deflection in thedirection of the light depending on the wavelength of light to reducethe variation in the final direction of the output light.

By including a beam stabilising component in the device the undesirabledependence of the output beam on operating conditions is advantageouslyremoved.

The aspects of the invention may be advantageously combined together. Inparticular, aspects which provide a small wavelength mismatch error, andaspects which provide a large acceptable wavelength mismatch error areadvantageously combined to provide a laser device with a broad operablerange with respect to operating conditions.

Example 1: Deep Ultraviolet Laser with λ_(peak, output)=225 nm

A first example of the invention is a device 1 which emits laser-likelight with a wavelength in the deep ultraviolet spectral range(wavelength between ≈200 nm and ≈300 nm) with a large operable rangewith respect to ambient temperature and operating mode. A schematicdiagram of an exemplary embodiment of the laser device 1 is shown inFIG. 15. The laser device includes a laser light source 2 which emitssource laser light 3 and a nonlinear optical component 4.

The laser light source 2 may be a semiconductor laser which emits lightwith a peak wavelength between a lower value of approximately 400 nm andan upper value of approximately 600 nm. The laser light source ispreferably a Fabry-Perot laser diode including Al_(y)In_(x)Ga_(1-x-y)Nsemiconductor materials and Al_(y)In_(x)Ga_(1-x-y)N light-emittinglayers (0≦x≦1; 0≦y≦1), and this is the case for the remainder of thisexample. The laser light source emits source light 3 with a peakwavelength of

_(peak, source)≈450 nm for an injection current equal to an operatingcurrent, J₀, when the temperature of the package of the laser lightsource is 25° C. (e.g. a temperature of the package of a laser diode maybe a temperature of the metal can which the laser diode is packagedinside, such as a standard 5.6 mm can package). The laser light sourcemay be configured to emit a near-diffraction limited beam (ascharacterised by a beam quality in all planes of the beam of M²<2), butin this example the laser light source is configured to emit anon-diffraction limited beam with M² approximately equal to valuesbetween 4 and 8. The laser light source in this example is configured tohave a dependence of peak wavelength on the temperature of the packagebetween −0.02 nm·K⁻¹ and +0.08 nm·K⁻¹ for a package temperature between−10° C. and 70° C. Preferably the dependence is between +0.01 nm·K⁻¹ and+0.06 nm·K⁻¹ for a package temperature between −10° C. and 70° C. Forthe remainder of this example, the value is approximately equal to +0.04nm·K⁻¹ for a package temperature between −10° C. and 70° C.

A nonlinear optical component 4 is configured to providefrequency-conversion of the source light 3 emitted by the laser lightsource 2. The nonlinear optical component is preferably a β-BaB₂O₄crystal configured to provide birefringent phase matched type I secondharmonic generation (SHG) of the light emitted by the laser lightsource. Preferably the β-BaB₂O₄ crystal is arranged such that thesurface of the crystal which the laser light propagates through as itenters the crystal (the entrance surface) and the surface of the crystalwhich the output light propagates through as it exits the crystal (theexit surface) are both approximately perpendicular to the laser lightbeam and that the laser light beam propagates inside the crystal at anangle (θ) approximately 63° away from the optic axis of the crystal withthe dominant electric field of the laser light perpendicular to theoptic axis. This configuration favourably provides high efficiency SHG(i.e. a high ratio of power of frequency converted output light dividedby power of laser light) at a wavelength of

_(PM)≈450 nm for a crystal temperature of approximately 25° C. For laserlight with other wavelengths, suitable values of θ are listed in FIG.16. The β-BaB₂O₄ crystal has a length, as measured parallel to thedirection of propagation of light inside the crystal, preferably between1 mm and 20 mm and more preferably between 5 mm and 15 mm. For theremainder of this example the length of the β-BaB₂O₄ crystal isapproximately 7 mm. The β-BaB₂O₄ crystal has a width and height,measured in two orthogonal directions perpendicular to the length andwhere width and height are not necessarily equal, preferably between 0.2mm and 5 mm, more preferably between approximately 0.5 mm and 2 mm, andmost preferably approximately 1 mm. Smaller values of width and heightprovide a nonlinear optical component with advantageously small heatcapacity.

Through new experiments the present inventors have determined, and showhere for the first time that for SHG using β-BaB₂O₄ with input laserlight wavelengths of

_(peak, source)≈449 nm (θ=64°), the dependence of

_(PM) on temperature for a β-BaB₂O₄ crystal as described and with alength of 7 mm is approximately equal to 0.025 nm·K⁻¹. Relevant data isshown in FIG. 17, where

_(M) for a 7 mm long β-BaB₂O₄ crystal was measured at 20° C., 30° C. and40° C. using a laser with tuneable emission wavelength and determiningthe laser wavelength which produced the highest second harmonic power,all while the direction of the source beam within the β-BaB₂O₄ crystalwas unchanged. Therefore, through use of laser light sources configuredto have dependence of peak wavelength on the temperature of the packageas described, an advantageously small wavelength mismatch error may beprovided over a wide range of operating conditions, such as ambienttemperature.

Referring again to FIG. 15, the source light 3 emitted by the laserlight source 2 is coupled into the nonlinear optical component 4 usingone or more optional optical components 10. Preferably the one or moreoptional optical components are configured to provide afrequency-conversion process with good efficiency and wide operablerange with respect to changes in operating conditions. The opticalcomponents may affect one or more of the following properties of thelaser light incident on the nonlinear optical component:

1. The direction of propagation of the beam.

2. The spatial position of the beam.

3. The spatial position of a waist of a focused (converging) beam.

4. A convergence half-angle of a focused (converging) beam.

For example, a collimating lens 11, a first cylindrical lens 12 and asecond cylindrical lens 13 may be disposed in the optical path betweenthe laser light source and the nonlinear optical component. The sourcelight 3 emitted by the laser light source is incident on a collimatinglens which collects the light into a partially or wholly collimated beamin at least one place of the beam. An example of a suitable lens is amoulded or precision polished glass aspheric lens with a focal lengthbetween 2 mm and 5 mm. In this example the collimating lens is a mouldedglass aspheric lens with a focal length of 3 mm. The collimated laserbeam is incident on a first cylindrical lens which focuses the light ina first plane of the beam, and then on a second cylindrical lens whichfocuses the light in a second plane of the beam (where the second planeof the beam is approximately perpendicular to the first plane of thebeam) where the first and second cylindrical lenses are configured sothat the beam converges in both the first and second planes of the beamas it propagates towards the nonlinear optical component 4. Preferablythe beam converges to a waist in at least one plane of the beam (i.e. aminimum beam width in a plane of the beam) within the nonlinear opticalcomponent. In an alternative option, a single lens (e.g. a sphericallens) may be used instead of first and second cylindrical lenses.

For this example, the convergence half-angle of the source light 3 inthe plane perpendicular to the phase matching axis is in the range0.4°-2.7° which may be provided using a first cylindrical lens 12 withfocal length between 30 mm and 200 mm, and is preferably approximately1.2°. For this example the convergence half-angle of the laser light inthe plane parallel to the phase matching axis is in the range 1°-12°which may be provided using a second cylindrical lens 13 with focallength between 5 mm and 75 mm, and is preferably approximately 6°. Thedominant electric field of the laser light is approximately parallel tothe phase matching axis of the nonlinear optical component.

As the laser light propagates through the nonlinear optical component,output light with wavelength λ_(peak, output)=

_(peak, source)/2≈225 nm is generated by SHG from the laser light.Unconverted, also referred to herein as residual, laser light 7 (i.e.any laser light which is not converted by SHG) and the output light 5both propagate out of the nonlinear optical component. The residuallaser light and the output light may be spatially overlapping. A filter6 which reduces the power of the residual laser light more than itreduces the power of the output light may be used. Most preferably,after a filter the power of the output light which exits the filter isgreater than the power of the residual light which exits the filter andis substantially coincident with the output light which exits thefilter. It is the power of the output light with a peak wavelength ofλ_(peak, output) which is considered when referring to the power of theoutput light of the laser device; the power of any residual laser lightwith peak wavelength

_(peak, source) present in the output beam is not included. The filtermay include one or more mirrors including a distributed Bragg reflector(DBR) which has a reflectivity greater than 90%, more preferably areflectivity greater than 99%, to the second harmonic light and areflectivity less than 1% to the residual laser light. Suitable DBRmirrors may be fabricated using layers of MgF₂ and LaF₃ on a UV fusedsilica substrate. The filter may also include a dispersive element, suchas a UV fused silica prism (e.g. a Pellin-Broca prism or an equilateralprism) arranged to spatially separate the output light and the residuallaser light. The output light 5 may be collected into a substantiallycollimated beam using one or more optional additional optics 9. Asuitable optic is a UV fused silica spherical lens. Any of the one ormore additional optics may have an antireflection coating which reducesthe reflection of light with wavelength λ_(peak, output).

The laser light source 2, the nonlinear optical component 4, andoptionally one or more of any optional optical components 6, 9, 10 areattached to a housing 8 which preferably provides good thermal contactbetween the laser light source and one or more nonlinear opticalcomponents, and further provides a compact laser device. A wide range ofmaterials are suitable for the housing, including metals and non-metals.The housing is configured to provide a rigid platform for housing thelaser light source, the nonlinear optical component and any otheroptical components, through selection of suitable material or materialsand its shape. Metal materials are particularly suitable owing to theirmechanical stiffness, thermal conductivity and ease of fabrication insuitable shape, for example by machining or casting. Examples ofsuitable metallic materials include aluminium, copper, zinc, steel andalloys based on the aforementioned materials. The housing provides afirst fixture point 30 to which the laser light source is attached. Thehousing further provides a second fixture point 31 to which thenonlinear optical component is attached. Preferably the thermalresistances between the laser light source and the first fixture point,between the nonlinear optical component and the second fixture point,and between the first fixture point and the second fixture points areall small. The distance between the first and second fixture points isless than 200 mm, and preferably less than 100 mm. Preferably thethermal properties of the housing are configured such that thedifference in temperature between the laser light source (a temperatureof the external package of the laser diode, or a temperature of asub-mount which the laser diode semiconductor chip is attached to) andthe β-BaB₂O₄ crystal is never more 40° C. during operation of the device(for example, from switch-on through to steady thermal state). Morepreferably said temperature difference is never more than 10° C.

For this example the housing is a continuous aluminium component withthermal resistance between the first fixture point and the secondfixture point approximately equal to 5 K·W⁻¹. The footprint of thehousing is preferably less than 200 mm×50 mm and more preferably lessthan 50 mm×50 mm, where “footprint” refers to a rectangle which thesurface of the housing fits wholly inside.

Preferably the one or more optical components 10 disposed on the opticalpath between the laser light source 2 and the nonlinear opticalcomponent 4 are configured to provide a laser device 1 with anadvantageously large operable range and a good frequency-conversionefficiency. According to information in the prior art, the operablerange of a laser device with the structure of the current example, withrespect to changes in ambient temperature, is expected to be very small.The tolerances of the laser device may be estimated using the commonlyused “SNLO” public domain software developed at Sandia NationalLaboratories for modelling of the efficiency of frequency-conversionprocesses in nonlinear optical materials. For a 7 mm long β-BaB₂O₄crystal configured to convert 450 nm laser light to 225 nm output lightby SHG at 25° C., the expected output power at different temperaturesmay be calculated using the 2D-mix-LP function of SNLO in the followingway: The crystal orientation for phase matching at the designtemperature is calculated (e.g. using SNLO's Qmix tool). The refractiveindices of the crystal are then computed over a range of temperaturesusing equations found in e.g. Kato et al. [Proc. SPIE 7582 1L (2010)]and the phase velocity mismatch (Δk) between the fundamental wavelengthand the second harmonic wavelength calculated. The refractive indicesand phase velocity mismatch are then used as inputs to the 2D-mix-LPmodel. This allows the temperatures at which the output power falls toe.g. 50% of the power at 25° C. (i.e. β=0.5) to be found. For the laserlight source and nonlinear optical component described in this example,the expected tolerance to change in temperature is ≈22° C.

For the device structure of the present example, however, experimentalmeasurements of the effect of the convergence half-angle of the sourcelaser light in the plane perpendicular to the phase-matching axis of aβ-BaB₂O₄ crystal on the frequency-conversion efficiency and on theoperable range are shown in FIG. 10, and show substantial unexpectedadvantage. Firstly, FIG. 10 shows the dependence of the power of theoutput light on the convergence half-angle. It is apparent that thepower of the output light is highest for smaller convergencehalf-angles, and is near a maximum for a convergence half-angle ofapproximately 0.6°; which is consistent with expectation from the priorart (e.g. US patent application number 2015/0177593A1). Secondly,however, FIG. 10 shows the experimentally-measured change to theβ-BaB₂O₄ temperature for which the efficiency of frequency conversiondrops to 50% of its maximum value (the full width at half maximum,FWHM). It can be seen that for a laser device according to this example,when the output power is maximised (convergence half-angle≈0.6°), themeasured tolerance to change in β-BaB₂O₄ temperature is ≈46° C., morethan double that predicted by the SNLO software. Thus, a laser deviceaccording to this invention has the unexpected and advantageouscharacteristic of reduced variation in output light intensity forchanges in temperature of the laser light source (and consequently thewavelength of the laser light) and for changes in temperature of theβ-BaB₂O₄ crystal.

Furthermore, there is a significant improvement in the tolerance to achange in β-BaB₂O₄ or laser light source temperature when theconvergence half-angle measure in the plane perpendicular to the phasematching axis is increased away from the convergence half-angle whichprovides the highest SHG conversion efficiency. Therefore, a deviceaccording to this example with the convergence half-angle measured inthe plane perpendicular to the phase matching axis which is larger thanthe convergence half-angle which provides optimum frequency conversionefficiency provides the unexpected advantage of a larger acceptablewavelength mismatch, and therefore a broader operable range for thelaser with respect to changes in temperature, changes to the injectioncurrent supplied to the laser or changes in the operating mode of thelaser.

For a convergence half-angle approximately equal to 1.2°, theexperimentally measured temperature tolerance (FWHM) of the β-BaB₂O₄crystal is 75.8° C. (FIG. 10), corresponding to a half-width at halfmaximum tolerance of 0.5×75.80° C.=37.9° C., which corresponds to anacceptable wavelength mismatch error (ω) of 37.9×0.025 nm=0.95 nm (usingthe value of 0.025 nm·K⁻¹ determined above; FIG. 17). For a targetoperable range of T_(range)=30° C. (e.g. 0° C. to 60° C., as requiredfor many applications of laser sources), ω/T_(range)≈0.03 nm·K⁻¹, asused in the first aspect of the detailed description. With the laserlight source 2 configured to have a dependence of peak wavelength ontemperature of approximately +0.04 nm·K⁻¹, the nonlinear opticalcomponent having a dependence of Λ_(PM) on temperature of approximately+0.025 nm·K⁻¹, |ΔΛ_(peak)/ΔT|_(T=T) _(j) −ΔΛ_(PM)/ΔT|_(T=T) _(j)|=(0.04−0.025) nm·K⁻¹=0.015 nm·K⁻¹<0.03 nm·K⁻¹, thereby satisfying thefirst aspect.

While this example has been described for a single nonlinear opticalcomponent, it should be appreciated that more than one nonlinear opticalcomponent can be used. For example, a pair of crystals arranged toprovide walk-off compensation may be used. While this example has beendescribed for the material β-BaB₂O₄, it should be appreciated that othermaterials can be used. For example the material may be SiO₂,Al_(y)Ga_(1-y)N (0.5≦y≦1), CsLiB₆O₁₀, LiB₃O₅, KBe₂BO₃F₂, Li₂B₄O₇,LiRbB₄O₇, MgBaF₄, Ba_(1-a)B_(2-b-c)O₄—Si_(a)Al_(b)Ga_(c) (0≦a≦0.15;0≦b≦0.10; 0≦c≦0.04; a+b+c≠0).

Example 2: Deep Ultraviolet Laser with λ_(peak, output)≈223 nm with aBeam Folding Optical Component

A second example of this invention is now disclosed. The second exampleis similar to the first example and common features may not be repeated.In this second example, which is illustrated in FIG. 18, a beam foldingoptical component 40 is disposed on the optical beam path between thelaser light source 2 and the nonlinear optical component 4. For example,the beam folding optical component may be a triangular prism withinternal angles of 45°, 45° and 90°. The prism may be arranged such thatthe source laser light 3 enters the prism approximately perpendicular tothe face opposite the 90° angle, undergoes total internal reflection ata second face of the prism, undergoes total internal reflection at athird face of the prism, and exits the prism with a direction ofpropagation parallel, but in opposite direction, to the direction of thelaser light incident on the prism (as shown in FIG. 18). Alternatively,the beam folding optic may be one or more mirrors which reflect thesource light.

The beam folding optical component 40 advantageously simultaneouslyenables good thermal contact between the laser light source 2 and thenonlinear optical component 4 and use of optical components 10configured to provide a convergence half-angle of source light 3incident on the nonlinear optical component which provides a largeoperable range of the laser device and good frequency-conversionefficiency. In particular, the beam folding optical component enablesthe laser light source and the nonlinear optical component to be placedin closer proximity to each other, thereby reducing the thermalresistance between the two components. As presented previously, this mayimprove the operable range with respect to changes in ambienttemperature, changes in the electrical current supplied to the laserlight source, and changes in the operating mode of the laser lightsource, and may reduce the time the laser device takes to arrive at asteady state thermal condition from a non-steady state condition.

The laser light source 2 may be a Fabry-Perot laser diode similar tothat in the first example, emitting source light 3 with a peakwavelength of

_(peak)≈445.9 nm at a package temperature of 25° C. (the temperature ofthe metal can containing the laser diode) for an injection current J₀,with a wavelength dependence on package temperature of approximately+0.04 nm·K⁻¹ and a spectral linewidth of approximately 1.7 nm. Thecollimating lens 11 may be a moulded aspheric lens. A first cylindricallens 12 and a second cylindrical lens 13 are configured to focus thesource light so that the source light converges in at least one plane ofthe beam as it propagates towards the nonlinear optical component 4. Thefirst cylindrical lens focuses the source light in the plane containingthe propagation direction of the beam and the direction perpendicular tothe electrical field vector of the laser light. The source lightincident on the nonlinear optical component has a convergence half-angleof approximately 1.2° in this plane. The second cylindrical lens focusesthe source light in the plane perpendicular to the first cylindricallens. The source light incident on the nonlinear optical component has aconvergence half-angle of approximately 6° in this plane. A triangularprism 40 as described earlier in this second example is used to “fold”the source light beam and is arranged such that the source light passesthrough the beam folding prism after passing through the firstcylindrical lens 12 and before passing through the second cylindricallens 13. The collimating lens, cylindrical lenses and the surface of thebeam folding prism opposite the 90° angle all have antireflectioncoatings for the wavelength of the source light. The nonlinear opticalcomponent is a single β-BaB₂O₄ crystal with a length of 7 mm configuredfor type I SHG, similar to that in the first example but where thesource light propagates inside the nonlinear optical component at anangle θ=64.5° away from the optic axis of the crystal. Thisconfiguration provides effective SHG for the source light. The dominantelectric field of the source light is parallel to the phase matchingaxis of the β-BaB₂O₄ crystal, and perpendicular to the optic axis of theβ-BaB₂O₄ crystal. The nonlinear optical component is configured so thatthe power of the second harmonic light is a maximum when the laserdevice was operated at 25° C. The power of the output light withλ_(peak, output)≈223 nm emitted by the device at a package temperatureof 25° C. is 29.8 μW for a laser diode injection current of 700 mA. Afilter 6 is used to reduce the power of the residual laser light 7 whichspatially overlaps with the output light 5 exiting the nonlinear opticalcomponent, as described in the first example. An optical component 9,which may include a lens, is positioned after the filter to collimatethe output light 5.

The laser light source 2, collimating lens 11, first cylindrical lens12, beam folding optical component 40, second cylindrical lens 13,nonlinear optical component 4, filter 6 and lens 9 are all attached to ahousing 8. In this example the housing is fabricated from a single pieceof aluminium alloy, but other materials may be used, as in the firstexample. The housing which contains the components described in thisexample has a footprint of 66.5 mm×50 mm and has a total heightincluding the optical components) of 12.5 mm. The housing is fitted witha lid which brings the total package size to 66.5 mm×50 mm×15 mm. Thedistance between the laser light source and the β-BaB₂O₄ crystal isapproximately 27 mm, a decrease of approximately 36 mm compared to alaser device which does not include a beam folding optical component.

The laser device 1 described in this example is demonstrated herein tohave exceptionally broad operable range with respect to changes inambient temperature. The laser device was operated with constantelectrical injection current supplied to the laser light source forambient temperatures between 3.3° C. and 55° C. The dependence of thepower of the output light on the ambient temperature is plotted in FIG.19. The data shows that the power of the output light 5 remains higherthan 55% of the power of the output light for the original designcondition (25° C.) throughout the range. This demonstrates the combinedeffect of (A) the advantageously small wavelength mismatch error in thelaser device, owing to (i) the laser light source 2 and nonlinearoptical component 4 being configured to provide a dependence of

_(peak) on temperature approximately equal to the dependence of the

_(PM) on temperature, and (ii) the housing 8 configured to provide goodthermal contact between the laser light source and nonlinear opticalcomponent; and (B) the advantageously large acceptable wavelengthmismatch error in the laser device, owing to (i) the collimating lens 11and first cylindrical lens 12 and second cylindrical lens 13 beingconfigured to provide a convergence half-angle for the source light 3significantly larger than for optimum frequency-conversion efficiency,and (ii) the laser light source being configured to emit source laserlight with a broad spectral bandwidth.

The laser device 1 is significantly different from, and advantageousover, laser devices in the prior art which may be configured to providedeep UV light (wavelength less than 260 nm). In particular, thecombination of laser light source 2 and nonlinear optical component 4 ina compact housing 8 (footprint=33 cm²), the provision of good thermalcontact between the laser light source and the nonlinear opticalcomponent, the absence of temperature stabilising components acting onat least one of the laser light source and the nonlinear opticalcomponent, and extremely large operable range of the laser device withrespect to ambient temperature are all individually novel over anyexamples in the prior art. In particular, the laser device producesapproximately twice as much deep ultraviolet output power as the systemdescribed in the recent prior art of Ruhnke et al. [Optics Letters 40, p2127 (2015)] (≈30 μW compared to ≈16 μW), while operating at a lowerlaser diode injection current (700 mA compared to 800 mA) and withoutthe need to place the β-BaB₂O₄ crystal in an oven to actively controlthe temperature. Compact laser devices emitting deep UV light with largeoperable range according to this invention enable new technology forsensing and disinfection.

The data in FIG. 19 shows that the power of the output light 5unexpectedly increases for temperatures lower than the design conditiontemperature even though the wavelength mismatch error increases as thetemperature is reduced below the design condition temperature. Forfurther advantageous improvements in the operable range of the laserdevice, therefore, it is preferred that the nonlinear optical component4 is configured to provide optimised frequency-conversion efficiency forthe wavelength of source light 3 emitted by the laser light source 2when the ambient temperature is higher than midway through the requiredoperable temperature range of the laser device 1. For example, it ispreferable in a laser device with operable range between 0° C. and 25°C. for the nonlinear optical component to be configured to provideoptimum frequency conversion efficiency at 25° C., rather than themidway temperature of 12.5° C. as would be naturally expected.

Example 3: Housing which Makes Use of Residual Blue Light to Heat BBOCrystal

A third example of this invention is now disclosed. The third example issimilar to the second example and common features may not be repeated.In this third example, which is illustrated in FIG. 20, the device 1includes a light absorbing component 50 which partially or fully absorbslight with the wavelength of the source light 3 emitted by the laserlight source 2. The light absorbing component 50 is attached to thenonlinear optical component 4, or alternatively it may be attached tothe housing 8 close to, or otherwise in thermal contact with, thenonlinear optical component.

The light absorbing component 50 may be a coating of a light absorbingmaterial applied to the nonlinear optical component or housing, such asfor example a metal oxide layer applied by anodising or painting. Thecomponent may be a separate piece which is attached to the nonlinearoptical component 4 or housing 8, for example by using a screw and anoptional heat transfer compound or by using an adhesive. Suitable heattransfer compounds may be metal oxide heat transfer compounds andsilicone heat transfer compounds. Suitable adhesives may be epoxyadhesives, cyanoacrylates and UV curing adhesives. The adhesive may be athermally conductive adhesive. In this example the light absorbing partis an anodised aluminium piece which is in physical contact with thenonlinear optical component 4 and attached to the housing using a UVcuring adhesive.

The housing 8 may optionally include one or more parts which increasethe thermal resistance between the light absorbing component 50 and thelaser light source 2. The housing may optionally include one or moreparts which increase the thermal resistance between the light absorbingcomponent and a heat sink. The optional one or more parts are preferablymade from a material which has a thermal conductivity less than 10W·m⁻¹·K⁻¹ and more preferably less than 0.5 W·m⁻¹K⁻¹. In this examplethe housing includes a component 100 which is constructed from ABSplastic which has a thermal conductivity of approximately 0.2 W·m⁻¹·K⁻¹and is attached to the aluminium housing using a UV curing adhesive. TheABS component 100 is positioned between the nonlinear optical component4 and both the laser light source and the heat sink and is a dominantfactor in determining the thermal resistance between both the lightabsorbing region and the laser light source and the light absorbingregion and the heat sink.

The residual light 7 propagating out of the nonlinear optical component4 may be made incident on the light absorbing component 50 through theoptional use of one or more additional optics 51, for example mirrors.These optional additional optics may be disposed at any of the followinglocations in the path of the residual light 7 beam: between thenonlinear optical component 4 and the filter 6; within the filter 6;between the filter 6 and the light absorbing component 50. For example,three mirrors may be positioned in the path of the residual light 7 beambetween the filter 6 and the light absorbing component 50, as shown inFIG. 20. Each mirror reflects the fundamental beam through an angle of90° so that the beam is made incident on the light absorbing component.Some or all of the light incident on the light absorbing part isconverted into heat, increasing the temperature of the nonlinear opticalcomponent. The ABS component 100 reduces the amount of heat whichreaches the laser light source 2 and the heat sink from the lightabsorbing component.

Example 4: Component to Reduce Change in Beam Direction and/or Position

A fourth example of this invention is now disclosed. The fourth exampleis similar to the first example and common features may not be repeated.In this fourth example, which is illustrated in FIG. 21, the device 1includes one or more optical components 60 which reduce the variation indirection or position of the output light beam 5 when operatingconditions are changed. As described above, a variation in the operatingconditions of a device according to any of the other examples in thisdisclosure may result in a change in the direction of thefrequency-converted output light (e.g. second harmonic light)propagating out of the nonlinear optical component 4, for example if anychange in operating condition causes a change in the wavelength of thesource light 3 emitted by the laser light source 2. Experimental datashowing this effect is displayed in FIG. 13. This data shows that theposition of the frequency-converted output light, measured on a CCD beamprofiler located a distance of approximately 8 cm from the nonlinearoptical component (measured along the propagation direction of theoutput light) changed when the wavelength of the source light 3 emittedby the laser light source 2 was changed. The change in position of thebeam on the CCD beam profiler corresponds to approximately a 0.5° changein the direction of the output beam for source light with wavelength447.9 nm compared to the direction of the output beam for source lightwith wavelength 449.6 nm. The wavelength of the source light 3 emittedby the laser light source 2, may change due to at least one of a changein the electrical current supplied to the laser light source, a changein the operating mode of the laser light source (such as a change induty cycle of pulse operation or a change between pulsed operation andcontinuous operation), and a change in the temperature of the laserlight source. Further experiments by the present inventors havedetermined that the direction of the output beam may also change whenthe temperature of the nonlinear optical component is changed. Using thesame experimental configuration as described for FIG. 13, the wavelengthof the laser light source was configured to be approximately 448 nm, andthe temperature of a β-BaB₂O₄ crystal (the nonlinear optical component)was changed. The direction of the frequency-converted (second harmonic)output beam propagating from the nonlinear optical component was foundto change by approximately 0.01° per Kelvin change in temperature of theβ-BaB₂O₄ crystal.

A variation in the direction of the output light may be a significantdisadvantage for applications of a device. For example, if the outputlight from a device is coupled into an optical system then a change inthe direction of the output beam (caused by a change in the operatingcondition of the device) may render the optical system inoperable. Forexample, if the output light is used in an optical system which isconfigured to measure the absorption of the output light as itpropagates through an analyte by using a detection component to measurethe power of the output light which has propagated through the analyte,a change in the direction of the output light from the device may causethe output light to propagate towards a location away from the detectioncomponent, and therefore make the optical system inoperable orunreliable.

In this fourth example, one or more optical components 60 are used toreduce the dependence of the direction of the output beam on theoperating conditions of the device. The one or more optical components60 may include a lens which is configured to reduce the variation indirection of the output beam. A lens located a distance, measured fromthe centre of the nonlinear optical component 4 along the propagationdirection of the output beam, approximately equal to the effective focallength of said lens may reduce the variation in direction of the outputbeam, as illustrated in FIG. 14 (b) wherein the output light propagatingin directions labelled 80, 81, 82 are deflected as they propagatethrough the lens such that after propagation through the lens theirdirections are preferably approximately parallel.

The lens 9 described in the first example may be configured to providethis effect (in which case the lens 9 is an example of an opticalcomponent 60). Although the variation in the direction of the outputbeam is reduced by said lens, the spatial position of the output beam isstill dependent on operating conditions. If optical component 60 is alens with effective focal length of approximately 30 mm, a variation indirection of the output beam 5 of 0.5° (as seen in FIG. 13), whichcorresponds to a change in spatial position of the output beam afterpropagation through the lens of approximately 260 μm. This change inspatial position may be reduced by including the one or more additionaloptical components 60 to act on the output beam 5. For example, a beamreducer (a beam expander used in reverse, i.e. to decrease the spot sizeof the output beam) may be used, whereby the reduction in variation ofthe output beam is then equal to the demagnification of the beamreducer. An example of a beam reducer is a piano-convex spherical lenswith a focal length of f=30 mm followed by a biconcave spherical lenswith a focal length of f=−6 mm arranged in a Galilean configuration.This results in a demagnification of ×5, and therefore reduces themagnitude of the spatial variation by a factor of five. The one or moreoptical components 60 may be disposed along the propagation direction ofthe output beam before and/or after the filter 6. In the case that twoor more optical components are used, optical components may be placedboth before and after the filter. Any of the one or more additionaloptical components may be the same optical components as the one or moreoptical components which comprise the filter.

Optical components 60 to reduce the variation in direction of the outputbeam in this fourth example may also include at least one of adiffraction grating, a prism and a concave mirror (for example withhyperbolic shape) as well as or instead of the lens.

Example 5: Laser with Wavelength Stabilising Component

A fifth example of this invention is now disclosed. The fifth example issimilar to the second example and common features may not be repeated.In this fifth example, which is illustrated in FIG. 22, the laser device1 includes a wavelength stabilising component 70. The wavelengthstabilising component 70 is configured to reduce a variation in thewavelength of the source light 3 emitted from the laser light source 2when operating conditions are changed.

In this example the wavelength stabilising component is a surfacediffraction grating, preferably a holographic diffraction grating, witha surface including an aluminium layer and with 3600 lines per mm.However, similar performance may be obtained using a grating with adifferent number of lines per mm, a grating with a different surfacematerial layers (e.g. silver) or a different grating type (e.g. a ruleddiffraction grating or volume Bragg grating). Furthermore, similarperformance may be obtained using a dichroic mirror or a band-passfilter combined with a mirror to reflect a narrow range of wavelengthsback towards the laser light source 2, which in this example is the samelaser diode as described in the first example.

The surface diffraction grating is arranged in a “Littrow” externalcavity diode laser configuration. Some or all of the residual light 7 isincident on the surface diffraction grating and this light is referredto as incident light 71. The surface diffraction grating is orientatedso that the incident light 71 is diffracted through preferably a firstorder diffraction (although higher order diffraction may be used) andsome of the diffracted light propagates back towards the laser diodealong the same path that the light emitted from the laser diodepropagated towards the surface diffraction grating, but in the oppositedirection. The diffracted light which propagates to the laser diodecauses the laser diode to preferentially emit a wavelength similar tothe wavelength of said light. The wavelength of the diffracted lightwhich propagates to the laser diode is dependent on the orientation ofthe surface diffraction grating. For a surface diffraction grating with3600 lines per mm the angle of incidence of the light onto the surfacediffraction grating is approximately 54.1° for a wavelength of 450 nm.The wavelength of the diffracted light which propagates to the laserdiode does not strongly depend on the operating conditions of the laserdiode. Therefore, the diffracted light acts to stabilise the wavelengthof light emitted by the laser diode,

_(peak, source), against variations caused by changes in an operatingcondition, e.g. change in ambient temperature, change in current and/orvoltage supplied to the laser diode, change in duty cycle of pulsedoperation of the laser diode, change between pulsed operation andcontinuous operation of the laser diode. Thus, the wavelength mismatcherror for the frequency conversion process (second harmonic generationin this example) may be reduced, compared with a device which does notinclude a wavelength stabilising component, thereby enabling a largeroperable range for the device 1 with respect to operating conditionsthan for a device which does not include a wavelength stabilisingcomponent. Additionally, the wavelength of the output light 5 of thelaser device 1, λ_(peak, output), may be also stabilised against changesin operating conditions because λ_(peak, output) is dependent on

_(peak, source). This may also provide a wider operable range for thelaser device, for example in applications where it is unacceptable forthe wavelength of the output light to vary significantly.

In this example, as illustrated in FIG. 22, the light emitted by thelaser diode is incident on the nonlinear optical component 4 and on thefilter 6 before it is incident on the wavelength stabilising component70, and then the diffracted beam which propagates back to the laserdiode is also incident on the filter and nonlinear optical componentbefore reaching the laser diode. One or more optical components 72 maybe added to collimate or focus the residual light 7 to form the incidentlight 71 which is incident on the surface diffraction grating.Preferably the incident light 71 forms a waist at the surfacediffraction grating. A suitable optic for the optical component 72 maybe a spherical lens with a focal length between 5 mm and 200 mm,preferably between 30 mm and 150 mm. Alternatively, a suitable systemincludes two cylindrical lenses each with a focal length between 5 mmand 200 mm wherein the first cylindrical lens focuses light in a firstplane and the second lens focuses light in a second plane which isperpendicular to the first plane. The focal lengths of the twocylindrical lenses may be different. The one or more optics may be thesame one or more optical components 9 in the first example forcollimating the output light 5 but this need not be the case. In thisexample the optical component 72 is a spherical lens. Some or all of thespecular reflection (zero-order diffraction) of the incident light 71from the diffraction grating may be used to heat the nonlinear opticalcomponent 4 as described in the third example. The portion of the powerof the incident light 71 which is diffracted in the diffracted light atthe surface diffraction grating may be in the range 1% to 99% and ispreferably in the range 50% to 99% for this example.

A laser device 1 has been constructed according to this fifth example.The device is comparable to the device 1 described in the second exampleunless noted otherwise. A surface diffraction grating (holographicdiffraction grating with 3600 lines per mm) was used as a wavelengthstabilising component 70, and a spherical lens with an effective focallength of 30 mm (for wavelength of λ=588 nm) was used as an opticalcomponent 72, as shown in FIG. 22. The surface diffraction grating waspermanently attached to the housing 8 with an orientation such that theincident light 71 was diffracted (first-order diffraction) along theoptical path back towards the laser diode. In this configuration thelight emitted by the laser diode had a peak wavelength

_(peak, source) of ≈447.1 nm, and the variation in emission wavelengthwith ambient temperature was approximately 0.005 nm·K⁻¹.

In an alternative configuration for this example (not shown in FIG. 22),the wavelength stabilising component 70 may be disposed on the opticalpath between the laser light source 2 and the nonlinear opticalcomponent 4. For example, the wavelength stabilising component may be asurface diffraction grating which is disposed so that light emitted bythe laser diode is substantially collimated by a collimating lens 11 andis then incident on said surface diffraction grating. In thisconfiguration the portion of the power of the light which is incident onthe surface diffraction grating which is diffracted as diffracted lightback towards the laser diode (e.g. in the first order of diffraction)may be in the range 1% to 99%, but is preferably in the range 5% to 20%for this example. The specular reflection (zero-order diffraction) ofthe light which is incident on the surface diffraction grating theninteracts with the focusing optics (e.g. first and second cylindricallenses 12,13) and passes into the nonlinear optical component 4 togenerate the frequency converted output light 5.

An aspect of the invention, therefore, is a laser device. In exemplaryembodiments, the laser device includes a light source configured to emita source light having a first peak wavelength, and a nonlinear opticalcomponent configured to perform a frequency conversion process thatconverts at least a portion of the source light into output light havinga second peak wavelength different from the first peak wavelength. Thelaser device includes a stabilization component configured to minimize amismatch error constituting a difference between the first peakwavelength of the source light and a wavelength for which the frequencyconversion process in the nonlinear optical component has a maximumvalue. The stabilization component includes a housing that houses thelight source and the nonlinear optical component. The laser device mayinclude one or more of the following features, either individually or incombination.

In an exemplary embodiment of the laser device, the housing is thermallyconductive between the light source and the nonlinear optical componentto minimize a temperature difference between the light source and thenonlinear optical component.

In an exemplary embodiment of the laser device, a distance betweenattachment points of the light source and the nonlinear opticalcomponent to the housing is less than 200 mm.

In an exemplary embodiment of the laser device, a thermal conductivityof material of the housing between the laser light source and thenonlinear optical component is at least 10 W·m⁻¹·K⁻¹.

In an exemplary embodiment of the laser device, a heat capacity of thehousing between the laser light source and the nonlinear opticalcomponent is less than 500 J·K⁻¹.

In an exemplary embodiment of the laser device, a heat capacity of thenonlinear optical component is less than 0.1 J·K⁻¹.

In an exemplary embodiment of the laser device, the laser device furtherincludes a heat sink within the housing.

In an exemplary embodiment of the laser device, the output lightincludes residual light having the first peak wavelength of the sourcelight. The laser device further includes an optical component configuredto direct the residual light to act as a heat source for heating thenonlinear optical component as part of minimizing the mismatch error.

In an exemplary embodiment of the laser device, the laser device furtherincludes an absorbing component in thermal contact with the nonlinearoptical component, wherein optical component directs the residual lightonto the absorbing component and the absorbing component absorbs theresidual light.

In an exemplary embodiment of the laser device, the laser device furtherincludes a filter that spatially separates the residual light from theportion of the output light having the second peak wavelength.

In an exemplary embodiment of the laser device, the laser device furtherincludes a focusing optical component that focuses the source light. Thefocusing optical component is configured to focus the source light tohave a first convergence half angle that is larger than a secondconvergence half angle that gives maximum output power.

In an exemplary embodiment of the laser device, the light source isconfigured to emit source light having a spectral linewidth of at least0.5 nm.

In an exemplary embodiment of the laser device, the laser device furtherincludes a beam stabilization optical component is configured tostabilize a direction and/or position of the output light.

In an exemplary embodiment of the laser device, the beam stabilizationoptical component comprises at least one of a lens, a diffractiongrating, a prism, or a mirror.

In an exemplary embodiment of the laser device, the stabilizationcomponent includes a wavelength stabilizing component configured toreduce a variation of the first peak wavelength of the source light.

In an exemplary embodiment of the laser device, the wavelengthstabilizing component comprises a diffraction grating.

In an exemplary embodiment of the laser device, the laser device isconfigured to have an operable range of ±10° C.

In an exemplary embodiment of the laser device, the nonlinear opticalcomponent is a frequency doubling component whereby the first peakwavelength of the source light is double the second peak wavelength ofthe output light.

In an exemplary embodiment of the laser device, the first peakwavelength is within a range from 400 nm to 600 nm, the second peakwavelength is within a range from 200 nm to 300 nm, and the light sourceis a laser diode.

Although the invention has been shown and described with respect to acertain embodiment or embodiments, equivalent alterations andmodifications may occur to others skilled in the art upon the readingand understanding of this specification and the annexed drawings. Inparticular regard to the various functions performed by the abovedescribed elements (components, assemblies, devices, compositions,etc.), the terms (including a reference to a “means”) used to describesuch elements are intended to correspond, unless otherwise indicated, toany element which performs the specified function of the describedelement (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein exemplary embodiment or embodiments of theinvention. In addition, while a particular feature of the invention mayhave been described above with respect to only one or more of severalembodiments, such feature may be combined with one or more otherfeatures of the other embodiments, as may be desired and advantageousfor any given or particular application.

Optionally, the device may also be arranged such that embodiments of theinvention may be utilized in just a part or sub-array of the entiredevice. Optionally, some or all of the multiple different embodimentsmay be utilized in different rows columns or regions of the device.

INDUSTRIAL APPLICABILITY

A laser device in accordance with the present invention may be used as adeep ultraviolet light source. Said light sources may be used influorescence sensors or absorption sensors.

What is claimed is:
 1. A laser device comprising: a light sourceconfigured to emit a source light having a first peak wavelength; anonlinear optical component configured to perform a frequency conversionprocess that converts at least a portion of the source light into outputlight having a second peak wavelength different from the first peakwavelength; and a focusing optical component that focuses the sourcelight; wherein the light source and the nonlinear optical component arepassively thermally conductive with each other such that a change intemperature of the light source results in a change in temperature ofthe nonlinear optical component, thereby minimizing a mismatch errorconstituting a difference between the first peak wavelength of thesource light and a wavelength for which the frequency conversion processin the nonlinear optical component has a maximum value; wherein thelaser device further includes a housing that houses the light source andthe nonlinear optical component; wherein the focusing optical componentis configured to focus the source light to have a first convergence halfangle that is larger than a second convergence half angle that givesmaximum output power of output light having the second peak wavelength;and wherein the light source is configured to emit source light having aspectral linewidth of at least 0.5 nm.
 2. The laser device of claim 1,wherein the housing is thermally conductive between the light source andthe nonlinear optical component to minimize a temperature differencebetween the light source and the nonlinear optical component.
 3. Thelaser device of claim 2, wherein a distance between attachment points ofthe light source and the nonlinear optical component to the housing isless than 200 mm.
 4. The laser device of claim 2, wherein a thermalconductivity of material of the housing between the laser light sourceand the nonlinear optical component is at least 10 W·m⁻¹·K⁻¹.
 5. Thelaser device of claim 2, wherein a heat capacity of the housing betweenthe laser light source and the nonlinear optical component is less than500 J·K⁻¹.
 6. The laser device of claim 2, wherein a heat capacity ofthe nonlinear optical component is less than 0.1 J·K⁻¹.
 7. The laserdevice of claim 2, further comprising a heat sink within the housing. 8.The laser device of claim 1, wherein the output light includes residuallight having the first peak wavelength of the source light; the laserdevice further comprising an optical component configured to direct theresidual light to act as a heat source for heating the nonlinear opticalcomponent as part of minimizing the mismatch error.
 9. The laser deviceof claim 8, further comprising an absorbing component in thermal contactwith the nonlinear optical component, wherein optical component directsthe residual light onto the absorbing component and the absorbingcomponent absorbs the residual light.
 10. The laser device of claim 8,further comprising a filter that spatially separates the residual lightfrom the portion of the output light having the second peak wavelength.11. The laser device of claim 1, further comprising a beam stabilizationoptical component configured to stabilize a direction and/or position ofthe output light.
 12. The laser device of claim 11, wherein the beamstabilization optical component comprises at least one of a lens, adiffraction grating, a prism, or a mirror.
 13. The laser device of claim1, further comprising a wavelength stabilizing component configured toreduce a variation of the first peak wavelength of the source light. 14.The laser device of claim 13, wherein the wavelength stabilizingcomponent comprises a diffraction grating.
 15. The laser device of claim1, wherein the laser device is configured to have an operable range of±10° C.
 16. The laser device of claim 1, wherein the nonlinear opticalcomponent is a frequency doubling component whereby the first peakwavelength of the source light is double the second peak wavelength ofthe output light.
 17. The laser device of claim 16, wherein the firstpeak wavelength is within a range from 400 nm to 600 nm, the second peakwavelength is within a range from 200 nm to 300 nm, and the light sourceis a laser diode.